In recent years, making a radio frequency (RF) circuit to be a monolithic microwave integrated circuit (MMIC) has been underway in a radio communication field using a millimeter waveband. However, since device capacity is poor in an ultra high frequency band such as a millimeter waveband, it is necessary to apply more current to make circuit characteristics having at least one of linear characteristics or frequency characteristics good, and power consumption tends to increase.
Meanwhile, there is also a trend to mount an RF circuit of a millimeter waveband on a compact portable information terminal. Therefore, an RF circuit of a millimeter waveband is required to lower power consumption, but once power consumption is lowered, at least one of the above described frequency characteristics and linear characteristics of a circuit is deteriorated. In order to achieve good communication, a distortion correction technique to correct a distortion component due to frequency characteristics or non-linear characteristics is necessary.
A configuration to provide a coupler in the final phase of the transmitting circuit of the receiving system to extract a transmitting signal, and to provide switches in the first phase of the receiving system to switch the extracted transmitting signal and the actually received signal, has been proposed as a distortion correction technique (for example, see patent literature 1).
FIG. 1 is a schematic configuration diagram of a conventional radio apparatus shown in patent literature 1. This radio apparatus is a multiband and multimode transmitting/receiving device supporting multiple frequency bands and multiple modulation schemes where modulation scheme A and modulation scheme B are mounted. This will be described using one modulation scheme A transmitting system and one modulation scheme B receiving system.
With radio apparatus 10, the transmitting system has a configuration to include pre-distortion section 11 that compares an input signal and a fed back signal and compensates for distortion, digital-to-analog (D/A) conversion 12 that converts a digital signal to an analog signal, modulating section 13 that converts a signal of low frequency component produced by D/A conversion 12 into a high frequency signal, power amplifier (PA) 14 that amplifies a high frequency signal to a desired power level, and coupler (CUP) 15 that takes out part of an amplified signal.
With radio apparatus 10, the receiving system has a configuration to include switch (SW) 16 that switches passing signals, low noise amplifier (LNA) 17 where a noise coefficient is low, demodulating section 18 that demodulates the original signal from a transmitted signal, and analog-to-digital (A/D) converter 19 that converts an analog signal to a digital signal.
An operation of radio apparatus 10 configured as above will be described below. In the transmitting system, after passing PA 14, a signal modulated by modulating section 13 is separated by CUP 15 into a signal to feed back to the receiving system and a signal to transmit as is, in order to compensate for the distortion of the input signal.
The signal separated as distortion compensation from CUP 15 is output to LNA 17 through SW 16 of the receiving system, and then demodulated by demodulating section 18. The demodulated received signal includes a distortion component, so that pre-distortion section 11 compares this received signal and the original transmitting signal, extracts a produced distortion component, and adds the extracted distortion component to the input transmitting signal to compensate distortion. This makes possible communication of high precision where a distortion component is removed.
By this means, by using a configuration of a radio apparatus as in patent literature 1, it is possible to compensate for distortion by extracting a distortion component of the transmitting system by the feedback through the receiving system and adding the extracted distortion component to the original signal.